The invention relates to a display device comprising at least one compartment containing an ionizable gas, walls of the compartment being provided with electrodes for selectively ionizing the ionizable gas, during operation, and comprising an electro-optical layer of an electro-optical material, and means for activating portions of the electro-optical layer.
Display devices for displaying monochromatic or color images include plasma-addressed liquid-crystal display devices, the so-called PALC-displays, which are preferably thin-type displays. PALC-displays are used as television and computer displays.
A display device of the type mentioned in the opening paragraph is disclosed in U.S. Pat. No. 5,596,431. The thin-type display device described in said document comprises a display screen having a pattern of (identical) so-called data storage or display elements and a plurality of compartments. Said compartments are filled with an ionizable gas and provided with electrodes for (selectively) ionizing the ionizable gas during operation. In the known display device, the compartments are mutually parallel, elongated channels (formed in a so-called channel plate), which serve as selection means for the display device (the so-called plasma-addressed row electrodes). The application of a voltage difference across the electrodes in one of the channels of the channel plate, causes electrons to be emitted (from the cathode), which electrons ionize the ionizable gas, thereby forming a plasma (plasma discharge). If the voltage across the electrodes in one channel is switched off and the gas de-ionized, a subsequent channel is energized. At the display-screen side of the display device, the compartments are sealed by a (thin) dielectric layer (xe2x80x9cmicrosheetxe2x80x9d) provided with a layer of an electro-optical material and further electrodes serving as the so-called data electrodes are column electrodes of the display device. Said further electrodes are provided on a substrate. The display device is formed by the assembly of the channel plate with the electrodes and the ionizable gas, the dielectric layer, the layer of the electro-optical material and the further electrodes.
A disadvantage of the known display device resides in that the plasma-discharge cycle of such display devices is not optimal.
It is an object of the invention to provide, inter alia, a display device having an improved plasma-discharge cycle.
To achieve this, the display device in accordance with the invention is characterized in that the ionizable gas comprises a carrier gas and an additional gas, said additional gas containing deuterium or deuterium hydrogen, or said additional gas containing hydrogen and a further gas having a higher molecular weight.
At the beginning of a plasma-discharge cycle in the display device, a plasma discharge is created in the compartment or in one of the compartments (for example the channels of a PALC-display) by applying a (relatively high) voltage pulse (the so-called xe2x80x9cstrobexe2x80x9d pulse) across the electrodes in the compartment. Charged particles are created in such a plasma discharge. The voltage across and the current through the discharge reach a stationary state (the so-called xe2x80x9csteady statexe2x80x9d) within a few us. After switching off the plasma discharge, the grey level of each display element is checked by applying a (relatively low) voltage across the corresponding further electrode (the data electrode or the column electrode). As a result, a part of the charged particles is drawn towards the (thin) dielectric layer (xe2x80x9cmicrosheetxe2x80x9d), which causes an electric field to be formed across the electro-optical layer. As a result, the electro-optical layer is charged like a capacitor until the complete data voltage is present across the layer, causing the transparency of the electro-optical layer to change (for example, it becomes more or less transparent). The degree of transparency is determined by the value of the data voltage. After the discharge has disappeared in the so-called xe2x80x9cafterglowxe2x80x9d, the compartment forms an insulator and the electro-optical layer remains charged. A new plasma discharge in the compartment acts as a reset of the display element.
In general, the display device comprises a number of compartments, each compartment including at least two electrodes for ionizing the gas.
Important parameters of the plasma-discharge cycle of the display device are the electric conductivity of the plasma discharge and the decay of the conductivity in the afterglow period. If the decay in conductivity of the plasma discharge takes place too slowly, the discharge may continue while a next data line is already being written, which is undesirable. Too rapid a decay of the conductivity also has adverse effects.
The (carrier) gas which is most commonly used in display devices of the type mentioned in the opening paragraph is helium (He). The ignition voltage of the plasma discharge can be reduced by adding small quantities of a gas (of the order of a few percent) to the helium. In general, the ionization potential of such gases is lower than that of helium. The resultant gas mixtures are referred to as Penning mixtures. A well-known additional gas is hydrogen (H2). The use of such gas mixtures does not only influence the ignition characteristic of the plasma discharge but also, for example, the current necessary to maintain the discharge (the so-called sustain current) as well as the afterglow characteristic of the discharge.
The inventors have recognized that the properties of the plasma-discharge cycle of the display device can be influenced by suitably selecting (the composition of) the additional gas. In a first embodiment of the invention, the additional gas comprises deuterium (D2) or deuterium hydrogen (HD), which causes the ignition and sustain voltages of the plasma discharge as well as the afterglow decay time of the plasma discharge to be influenced. The addition of gases having a larger effective cross-section and a higher molecular weight, such as D2 or HD, additionally causes the out-diffusion of the additional gas to be effectively reduced. The small molecule hydrogen has a relatively high diffusion rate through the (glass) walls of the display device, and diffusion takes place, in particular, through the dielectric layer (xe2x80x9cmicrosheetxe2x80x9d) which seals the compartment and which generally has a thickness of approximately only 50 xcexcm. In a second embodiment of the invention, the additional gas consists of hydrogen and a further gas having a higher molecular weight, resulting in an additional parameter by means of which the properties of the plasma discharge can be influenced.
In a preferred gas mixture for a display device in accordance with the first embodiment of the invention, helium (He) or nitrogen (N2) is the carrier gas, and the additional gas comprises either deuterium (D2) or deuterium hydrogen (HD). Further, nitrogen (N2) or xenon (Xe) may be added to the additional gas.
In a preferred gas mixture for a display device in accordance with the second embodiment of the invention, helium (He) is the carrier gas, and the additional gas comprises hydrogen (H2) and nitrogen (N2), or hydrogen (H2) and xenon (Xe). Further, hydrogen (H2) may be at least partly replaced by deuterium (D2) or deuterium hydrogen (HD).
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.